Plasmonic laser puts the squeeze on light

Researchers at the University of California at Berkeley claim to have created the smallest semiconductor laser ever. The new nanoscale device can generate light in a space just 5 nm in size, which is 100 times smaller than the spot produced by conventional lasers. The feat could pave the way for a host of applications, including optical computers that use light instead of electrons to process information, biosensors and nanometre-sized photonic circuits.

Normally, light cannot be focused to a spot smaller than half its wavelength – something known as the diffraction limit. However, in recent years, scientists have succeeded in compressing light down to the nanoscale by coupling it to the electrons that oscillate collectively at the surface of metals – called surface plasmons. The resulting excitations of light and electrons are known as "surface plasmon polaritons" or SPPs.

Previous attempts to exploit SPPs to make nanoscale plasmonic lasers failed because the inherent resistance of metals absorbs the SPPs, causing them to dissipate almost immediately after they are generated. This effect becomes worse the tighter the light is bound to the surface.

Now, Xiang Zhang and colleagues have overcome this problem by constructing a hybrid device consisting of a cadmium sulphide semiconductor nanowire separated by a 5 nm thick insulating layer from a metallic silver surface. This structure – dubbed a "hybrid plasmonic waveguide" by the researchers – can concentrate light into an area as much as 100 times smaller than a diffraction-limited spot. And, because it is non-metallic, it poses little resistance so that SPPs can survive for longer.

The researchers can then amplify the SPPs present by shining light onto the structure. "We are able to do this because the nanowire essentially acts an amplifier for nanoscale light," team member Rupert Oulton told our sister website physicsworld.com. "This is something that scientists have been trying to achieve for about six years now and is an important milestone for turning the science of nanoscale light into technology."

The result is all the more exciting because it has been demonstrated with semiconductor materials, which are fully compatible with modern electronic device engineering, he added.

Sniffing out single molecules

The most interesting applications to come out of this research will be those that take advantage of the nanoscale light produced. For example, the interactions between light and matter could be strengthened, which means that very weak effects might be observable. This could come in handy for detecting single molecules, allowing for extremely sensitive biodetection, said Oulton.

This is something that scientists have been trying to achieve for about six years now and is an important milestone for turning the science of nanoscale light into technology.
Rupert Oulton, University of California, Berkeley

"We have also shown that the plasmon laser is very efficient so it could operate in a similar way to a conventional laser," he explained. "The ultra-small size of the light would increase the speed of optical telecommunications, while the compact dimensions of the device would allow you to pack and modulate thousands of these tiny light transmitters onto a single chip." Such schemes are promising since computers are fast reaching the speed limitations of electronics and will need to move to optics for a significant leap forward, he said.

The new device follows hot on the heels of another nanolaser, the "spaser", developed by researchers from Purdue, Cornell and Norfolk State universities. Here, a dye coupled to gold spheres just 44 nm across immersed in solution generates surface plasmons when exposed to light.

Towards a practical device

The UC Berkeley team now plans to improve its technology by exciting its laser using electrical current instead of light so that the device is more practical to use. "This will not be that difficult – optical injection is far less efficient than electrical, so the hard part is done," said Oulton.

"This is a nice work indeed," commented spaser co-inventor Vladimir Shalaev of Purdue University. "And it's interesting that the Berkeley and the NSU-Purdue-Cornell work on nanolasers both came out almost at the same time, at a moment when the community is preparing to celebrate the 50th anniversary of the invention of the laser next year. Both types of nanolaser represent important breakthroughs and we can expect many applications in nanophotonics, sensing, and other fields of science and tech."